The synapsins are a family of proteins that play a crucial role in the regulation of neurotransmission and in neurodevelopment. Single genes encode synapsins I and II. The synapsin I gene gives rise to two alternatively spliced mRNAs, which in turn, give rise to the synapsin Ia and Ib protein. The synapsin II gene gives rise to two alternatively spliced mRNAs, which in turn, give rise to the synapsin Ia and IIb protein (1).
The synapsin Ia, Ib, IIa, and IIb proteins are all found in the brain, where they are specifically localized to the presynaptic region of neurons, and coat synaptic vesicles. There is considerable experimental evidence that synapsins regulate the release of neurotransmission by becoming phosphorylated through the action of specific protein kinases (2).
There is also compelling evidence from several biological systems that synapsins play a critical role in axonal outgrowth, synapse formation, and synapse maintenance (3-8). Synapsins are potent stimulators of synapse formation, and may be useful as therapeutic agents for neurodegenerative diseases.
To date, no satisfactory candidate susceptibility gene for schizophrenia has been identified within the region of 22q12-13 human chromosome 22, although several studies report that this region has a statistically significant chance of containing such a gene. Since synapsins play an integral role in neurotransmitter release and synaptogenesis, mutations in the gene therefor could explain many of the defects observed in neurotransmission and in synapse formation in schizophrenia. Schizophrenia is a psychiatric illness that affects approximately 1% of the population worldwide. The illness is characterized by positive symptoms such as hallucinations, delusions, bizarre behavior, and thought disorder, as well as negative symptoms such as lack of motivation, social withdrawal, and apathy. Unfortunately, there is no objective laboratory test for schizophrenia, and the diagnosis is made by clinical interview. Current medication is effective for treating the positive symptoms of the disease, with little effect on the negative symptoms. Although the cause of schizophrenia is unknown, the disease has a strong genetic component. Research into the genetics of schizophrenia reveals that this disease is heterogeneous and is a "complex genetic" disease--that is, several genes may be involved in the etiology of this disease. In identical twin studies, where one twin is affected by the disease, the other twin has a 50% chance of succumbing to the disease. Since identical twins possess identical genes, the penetrance of the disease in the setting of mutated genes is only 50%. Thus, an unknown environmental component as well as the presence of specific mutated genes, is required for the generation of schizophrenia.
Biochemical and cytological studies suggest that in schizophrenia, defects in neurotransmission and synapses occur. Abnormal neurotransmission affecting the dopamine, serotonin, glutamine, .gamma.-aminobutyric acid, and cholecystokinin systems have been reported in schizophrenia (18). Neurodevelopmental abnormalities are also strongly implicated in schizophrenia, with reports of defects in neuronal cytoskeleton (19), neuronal cytoarchitecture and migration (20), cellular polarity (21), and synaptic pruning (22). Thus, in schizophrenia, at least two processes appear to be aberrant: neurotransmission and neuronal development, primarily affecting the later stages of synapse formation.
These studies indicate that a candidate susceptibility gene for schizophrenia should be expressed in the brain, and would likely play a role in synapse formation and neurotransmission. Furthermore, a candidate susceptibility gene should be genetically linked to schizophrenia. As indicated before, several research groups have independently identified a region on chromosome 22 that appears to possess a candidate susceptibility gene for schizophrenia (9-17). To date, a susceptibility gene has not been identified in this region.
Schizophrenia is a common disease, with a world-wide prevalence of 1%, affecting families of all races and socioeconomic groups, and consuming a significant portion of all medical and social expenses.
Since there is a continuing need for new therapies for such diseases, efforts have been devoted to the characterization and elucidation of the genes for synapsins, and their various attendant uses. The susceptibility gene for schizophrenia has great commercial value both as a diagnostic reagent and for developing new treatments for this disease.